The present invention relates to a semiconductor device comprising an interlaminar insulating layer disposed on a wiring layer and composed of a porous layer or an SOG layer, and a method of producing the same.
As examples of the material forming an interlaminar insulating layer in a semiconductor device, there have conventionally been known organic and inorganic materials. An interlaminar insulating layer formed of an organic material is disadvantageously poor in heat resistance although its relative dielectric constant is relatively low, while an interlaminar insulating layer made of an inorganic material is disadvantageously high in relative dielectric constant although its heat resistance is excellent.
As the interlaminar insulating layer formed of an organic material low in relative dielectric constant, there is known an aggregate layer comprising organic silanol condensate particulates each having a structure as shown in FIG. 18. More specifically, each organic silanol condensate particulate is arranged such that silicon-alkyl group bonds (organic units) are uniformly dispersed, in the molecular level, in silicon-oxygen bonds (inorganic units) and that silanol groups (Si--OH) are present on the surface thereof. In FIG. 18, R is an alkyl group such as CH.sub.3, C.sub.2 H.sub.5, C.sub.6 H.sub.5 or the like.
An interlaminar insulating layer of prior art is formed in the following manner. A TEOS derivative having an alkyl group as a substituent group (in which silicon-alkyl group bonds are substantially uniformly dispersed, in the molecular level, in silicon-oxygen bonds) is hydrolyzed and then dehydrated and condensed to prepare a silica sol, and the silica sol thus prepared is applied onto a semiconductor substrate and then thermally treated, thus forming an interlaminar insulating layer.
Each of the organic silanol condensate particulates forming an insulating layer composed of an SOG layer of prior art is arranged, as mentioned above, such that silicon-alkyl group bonds (organic units) are uniformly dispersed, in the molecular level, in silicon-oxygen bonds (inorganic units), and the silicon-alkyl group bonds are less stable than the silicon-oxygen bonds. Therefore, a variety of problems arise as set forth below. However, the following description will first discuss, as a premise, a process of forming a contact hole in an insulating layer composed of an SOG layer formed on a semiconductor substrate having a metallic layer.
As shown in FIG. 19 (a), the first layer of SiO.sub.2 102 having a thickness of 50 nm is deposited, by a CVD method, throughout the surface of the first metallic wiring layer of aluminium 101 formed on a semiconductor substrate 100. After an interlaminar insulating layer 103 composed of an SOG layer is deposited on the first layer of SiO.sub.2 102, the second layer of SiO.sub.2 104 having a thickness of 100 nm is deposited on the interlaminar insulating layer 103 by a CVD method as shown in FIG. 19 (b).
Then, a resist pattern 105 composed of organic matter is formed on the second layer of SiO.sub.2 104 as shown in FIG. 19 (c). Then, using the resist pattern 105, the first layer of SiO.sub.2 102, the interlaminar insulating layer 103 and the second layer of SiO.sub.2 104 are etched to form a contact hole 106 as shown in FIG. 20 (a).
As shown in FIG. 20 (b), the resist pattern 105 is then removed using oxygen plasma.
As mentioned earlier, the silicon-alkyl group bonds are less stable than the silicon-oxygen bonds. Accordingly, oxidative decomposition of the silicon-alkyl group bonds proceeds deep in the lateral wall of the contact hole 106 in the interlaminar insulating layer 103. Side-etching disadvantageously occurs in those portions of the interlaminar insulating layer 103 exposed to the contact hole 106 as shown in FIG. 20 (b).
Due to the heat from a surface thermal treatment conducted just before the second layer of metallic wiring 107 is embedded in the contact hole 106 (See FIG. 21), moisture is generated from the interlaminar insulating layer 103 as shown in FIG. 20 (c). Disadvantageously, the interlaminar insulating layer 103 absorbs moisture to increase the dielectric constant, and the surface of the first metallic wiring layer of aluminium 101 is oxidized to increase the contact resistance.
When the second layer of metallic wiring 107 is deposited as shown in FIG. 21, a void 108 is generated in the second layer of metallic wiring 107 due to the side-etching above-mentioned. This disadvantageously causes the second layer of metallic wiring 107 to be reduced in thickness or to be disconnected.
There is desired an interlaminar insulating layer low in relative dielectric constant yet assuring the heat resistance. In this connection, the following technique is proposed to form an interlaminar insulating layer composed of a porous layer.
FIG. 22 shows a sectional structure of a semiconductor device disclosed by Japanese Patent Publication No. 7-46698. As shown in FIG. 22, a metallic wiring 111 is formed on a semiconductor substrate 110 and an interlaminar insulating layer 112 composed of a porous layer is formed throughout the surface of the semiconductor substrate 110 including the metallic wiring 111.
FIG. 23 shows a sectional structure of a semiconductor device disclosed by Japanese Patent Publication No. 6-12790. As shown in FIG. 23, a metallic wiring 121 is formed on a semiconductor substrate 120 and a first SOG layer 122 is formed, by a CVD method, throughout the surface of the semiconductor substrate 120 including the metallic wiring 121. Then, an organic porous layer 123 is formed on the first SOG layer 122, and a second SOG layer 124 is then formed on the porous layer 123 by a CVD method. These first SOG layer 122, the organic porous layer 123 and the second SOG layer 124 form an interlaminar insulating layer.
In any of the publications above-mentioned, however, no particular description has been made of how to form the porous layer. Accordingly, porous layer forming methods discussed in the following papers are now taken into consideration.
As a first porous layer forming method, there is mentioned a method disclosed by IEEE Transactions on components, hybrids, and manufacturing technology, Vol. 15, No. 6 p. 925 (1992). More specifically, there is formed an organic high polymer layer composed of a co-polymer comprising an organic high polymer precursor high in heat resistance and an organic high polymer precursor low in heat resistance, and the organic high polymer layer thus formed is then thermally treated to decompose the organic portion low in heat resistance, thus forming a porous layer composed of an organic high polymer material.
As a second porous layer forming method, there is mentioned a method disclosed by Makromol. Chem., Macromol. Symp. 42/43, 393 (1991). More specifically, a silica film containing an organic high polymer is formed from a mixture solution of a silanol sol and an organic high polymer, and then thermally treated to pyrolytically decompose the organic high polymer, thus forming a porous layer composed of an inorganic material.
However, when forming an embedded wiring in an interlaminar insulating layer composed of a porous layer, the following problem arises. When a concave groove for an embedded wiring is formed in the interlaminar insulating layer composed of a porous layer and a wiring material is embedded in the concave groove thus formed, the wiring material enters holes in the porous layer. This disadvantageously deteriorates the insulating properties of the interlaminar insulating layer composed of a porous layer. Further, the wiring layer becomes uneven at the lateral side thereof to deteriorate the resistance to electromigration of the wiring layer. This disadvantageously deteriorates the stability of electric characteristics of the semiconductor device. It is therefore difficult to form an embedded wiring in the interlaminar insulating layer composed of a porous layer.
According to the first porous layer forming method, to thermally treat the organic high polymer layer, the semiconductor substrate is required to be maintained at a temperature of 275.degree. C. for about nine hours. This disadvantageously takes much time for a treatment of making the organic high polymer layer porous. Accordingly, it would be considered to form a porous layer in a short period of time by raising the temperature of the heat treatment. However, if the substrate is thermally treated at a temperature of not less than 400.degree. C., the organic high polymer is inevitably decomposed. Thus, raising the treatment temperature could not be a radical solving means.
According to the second porous layer forming method, the semiconductor substrate is required to be maintained at a temperature of 600.degree. C. for about 24 hours. This also disadvantageously takes much time. Accordingly, it would also be considered to form a porous layer in a short period of time by raising the temperature of the heat treatment. However, if the treatment temperature is raised, the glass component of the inorganic material is molten and the molten glass component closes holes in the porous layer. Thus, this could neither be a radical solving means.
As discussed in the foregoing, each of the porous layer forming methods of prior art cannot be utilized for forming an interlaminar insulating layer composed of a porous layer in the production process of a semiconductor device.
In view of the foregoing, it is a first object of the present invention to make provision such that an embedded wiring can be formed in an interlaminar insulating layer. It is a second object of the present invention to make provision such that an interlaminar insulating layer composed of a porous layer can be formed at a -low temperature under a normal pressure in a short period of time. It is a third object of the present invention to provide an insulating layer which is hardly side-etched by oxygen plasma used for removing the resist pattern, and which hardly generates moisture even though heated, thus not only preventing the upper metallic wiring of the semiconductor device from being reduced in thickness and from being disconnected, but also reducing the contact resistance between the lower and upper metallic wirings to restrain the relative dielectric constant from being increased due to moisture absorption.